Your thread reminded me of how my trainer back home had me teach my horse how to leg yield. I'd almost forgotten, as I do it differently now. So thanks for the flashback!
I taught the leg yield the same way that it sounds like you are, beginning first with the turn on the forehand. So you know how you restrict/block the shoulders during the turn? Progressing it into leg yield is allowing the shoulders to move in the same direction as the hind end.
The exercise I used is this (let's say we're on the right rein):
1. Halt straight, not too near the wall/fence/edge of the arena.
2. Bend the horse slightly towards the wall/left, and apply the aids for a couple steps on turn on the forehand.
3. Maintain the asking left aids, but soften your right aids to allow the horse to step sideways, crossing both their front and hind legs equally. The wall of the arena will help prevent them from avoiding going sideways and going straight instead. At first, you may need to open your right rein a bit (but do not open your right leg!) to show or guide the horse where you want it to go.
4. Praise/reward after a few steps of leg yield, halt and finish your turn on the forehand so that you are now facing the opposite direction you started on (now you would be on the left rein) and repeat the steps going in this direction!
You say that your horse is trying to keep turning. I would make sure that your asking leg is not continuing to push at the same place way behind the girth and that it stays right at the girth or just lightly behind. Use your other aids to show her where she is to go and make sure that you are not contorting you body from trying so hard to make her understand. (I used to practice shoulder-in bareback when I was first learning it because if did not sit up straight and got to sitting sideways and crooked, I would slide off.)
How I teach leg yield now is by teaching the horse to leg yield/sidepass in-hand. I create the bend, aids and voice command on the ground and then it all translates easily when I am in the saddle.
I reccomend taking some lessons from your trianer to help you with this as it is very helpful to have an experienced person on the ground to coach you, especially when that person has taught multiple horses how to leg yield and is aware of the common sticky spots.
"He doth nothing but talk of his horses."